In recent years, a near-infrared (NIR) spectroscopy method, a powder X-ray diffraction method, or a solid CMR method has been used as a method for physically measuring a solid sample. However, these measuring methods are disadvantageous, for example, in that a quantitative analysis cannot be performed without references, that the detection limit is high, that the signal strength depends on a crystal size, and that a specific crystal form, such as an amorphous form, cannot be detected.
On the other hand, a proton NMR (PMR) method has been widely used as a means for measuring a sample dissolved in a solution.
Protons have a high natural abundance ratio, and are higher in detection sensitivity than other elements, and hence are suitable for analysis.
The PMR method is performed such that protons placed in a static magnetic field are irradiated with an RF magnetic field, and that the energy change of the protons resonant with the RF magnetic field is recorded as an electric signal.
The principle of the proton nuclear magnetic resonance is as follows.
An atomic nucleus has a minute magnet (spin magnetic moment). The spin magnetic moment of a proton placed in a non-magnetic environment faces a random direction. When this is placed in a static magnetic field (+Z direction), the magnetic moment starts Larmor precession at a slightly oblique angle with respect to an axis H0 of the magnetic field. Its angular velocity ω0 is proportional to the magnetic field strength H0.ω0=(γ/2π)H0
The symbol γ is called a gyromagnetic ratio, and is an intrinsic constant of a nuclide. The rotational phase is in disorder, and is uniformly distributed in a vertically conical shape.
Groups in the up-direction are excessive in the magnetic field, and the resultant vector M of these groups follows the +Z direction. These are spin groups that are treated as a subject of the NMR phenomenon.
To obtain an NMR signal, a radio wave having the same angular velocity as that of the precession movement is irradiated from the X axis.
As a result, the spin groups absorb the energy of the radio wave to bring about vector components Mx and My. An NMR signal can be obtained by detecting the vector component My by use of a receiver coil placed in the y-direction.
If the radio wave is a pulse, reference is made to as the irradiation of, for example, a 90-degree pulse or a 180-degree pulse. Tilt angle values, such as 90 degrees or 180 degrees, are specified by an angle at which a spin is inclined from the +Z direction. The tilt angle can be changed by pulse width (microseconds) and pulse strength.
An electric current detected by the receiver coil is called “FID” (Free Induction Decay), and its strength is maximized when irradiated pulses are cut, and is attenuated with the lapse of time.
The orbit of a magnetic moment M during a relaxation process is provided by recording the strength of an electric current produced by a detector coil when a 90-degree pulse is irradiated. This measuring method is called an IR (Inversion Recovery) method.
Especially, a pulse sequence of (180°-τ-90°)n is often used in this method, and is also applied to, for example, a study of the properties of a compound or to MRI in the medical field.
The IR method using this pulse sequence of (180°-τ-90°)n will be explained.
The directions of magnetic moments of proton spins in a steady state coincide with the Z direction. Therefore, the resultant vector thereof is present at +Zo. The irradiation of a 180-degree pulse thereonto allows the direction of the proton spins to make a 180-degree inversion and hence to face the −ZO direction.
To record this state, a 90-degree pulse is irradiated after the lapse of τ seconds after completing the irradiation of the 180-degree pulse. The vector takes a 180°+90° position (270-degree position) if it is immediately after the irradiation of the 180-degree pulse. Therefore, the NMR signal becomes a maximum minus signal.
If the pulse sequence of (180°-τ-90°)n is irradiated a plurality of times while changing the value τ so as to record a change in the NMR signal with respect to the value τ, a longitudinal relaxation curve that results from a single proton and is drawn according to the following equation can be obtained:Signal strength y=[1−2exp(−τ/T1)]where T1 is the time during which the nuclear spin facing the −ZO direction returns to the initial state of facing the +ZO direction, and is called the spin-lattice relaxation time or the longitudinal relaxation time (T1).
FIG. 7 graphs this longitudinal relaxation curve.
The value of the longitudinal relaxation curve recovers to be signal strength of zero after 0.693T1 seconds, and reaches a substantially saturated state after 5T1 seconds.
The value of T1 gives an intrinsic value to a proton environment, and hence can be used to obtain information about a molecule. For example, the value of T1 reflects a molecule-to-molecule distance in powder, and can be used as information showing a difference in the molecular structure.
In a process in which the NMR signal is received, a coil-induced electric current disappears with the progression of transverse relaxation immediately after finishing the irradiation of the 90-degree pulse. An FID (Free Induction Decay) signal is the one that records this, and is a time-domain spectrum signal when the abscissa axis shows time. The FID signal is attenuated by exp(−/T2) where T2 is called the transverse relaxation time depending on an environment in which protons are placed, and is a piece of chemically important information.
A so-called NMR spectrum whose abscissa axis shows a frequency domain can be detected by subjecting this FID signal to a Fourier transform.    Non-patent literature 1: Journal of American Chemical Society 121, 11554-11557 (1999)    Non-patent literature 2: Australian Journal of Soil Research 38, 665-683 (2000)    Non-patent literature 3: Solid State Nuclear Magnetic Resonance 15, 239-248 (2000)    Non-patent literature 4: Journal of Chemometrics 13, 95-110 (1999)